The present invention relates to a color correction conditions calculating method, a color correction method, a color correction conditions calculation apparatus, a color correction apparatus and a storing medium for correcting the color of a color original image such as an image recorded on a color light-sensitive material (a color negative film and a color reversal film) and a digital image obtained with a digital camera.
Even if printing to a photographic paper is conducted from an image recorded on a color light-sensitive material which has been subjected to photographic processing such as a color negative film or color reversal film by means of a conventional method such as a LATD control, images having poor color balance have frequently been produced depending upon the kind of light-sensitive material, photographing light source and photographic processing. In addition, even when an image is displayed on a monitor or outputted as a hard copy from a color digital image obtained by photographing a color light-sensitive material which has been subjected to photographic processing, images having poor color balance have frequently been produced depending upon the kind of light-sensitive material, photographing light source, photographic processing and image-reading device. In addition, even if an image is displayed onto a monitor image on a frame of a color negative film as it is from a color original digital image obtained through photographing by means of a digital camera or outputted as a hard copy, images having poor color balance have frequently been produced depending upon the kind of digital camera, photographing light source and a photographed scene. As described above, if a color original image is used as is, images having poor image quality are frequently produced. Therefore, an original image may be reproduced by correcting color by means of various methods. However, due to conventional color correction methods, images having poor color balance have still frequently been produced.
In addition, in order to display an image on a monitor from color original image through a digital original image or in order to output an image as a hard copy, as a method of correcting color, it is ordinary that a method of using chromaticity is known. According to this technique, a standard color chart (wherein chromaticity is selected in such a manner that a color solid can be constituted) is prepared by a coloring material of a color original image. Chromaticity such as Lab, Luv and XYZ, is measured by means of a chromaticity-meter and the prepared color chart is imagewise read by means of an image inputting device such as a digital camera or scanner for obtaining three primary color information. From the resulting chromaticity and three primary colors information, a three dimension function F.sub.1 which converts the three primary information in an image inputting device to chromaticity is calculated. On the other hand, an image can be reproduced by means of a monitor or a hard copy as a color chart from conventional three primary color information selected in such a manner that the color solid can be constituted by means of a monitor or an image outputting device. The chromaticity of the color chart reproduced can be measured by means of a chromaticity-meter so that a three dimensional function F.sub.2 wherein the resulting three primary color information is converted to the three primary color information of the image outputting in view of chromaticity was measured. By storing aforesaid functions, converting digital inputting images obtained by image-reading actual color original images with an image inputting device into the chromaticity by means of function F.sub.1 and by subsequently converting the chromaticity to a digitally outputted image by means of the stored function F.sub.2, color is corrected, displayed on a monitor or outputted as a hard copy.
However, according to the above-mentioned technology, it was difficult to prepare a standard color chart by means of a coloring material of the color original image and the form of the color solid of the three primary color information and that of the color solid of the chromaticity such as the Lab, Luv and XYZ are remarkably different. Therefore, the three dimensional function F.sub.1 and the three dimensional function F.sub.2 are subjected to approximate operation processing with the result that errors tend to occur. Accordingly, images cannot be converted accurately. Specifically, due to aforesaid approximate operation processing, there was a problem in neutrally reproducing poor color pixels. Specifically, when a color original image is a color negative film the colors of the color mask are mixed when a negative film is reversed to be a positive image. Therefore, it is impossible to reverse the negative film to a positive image satisfactorily. Accordingly, aforesaid method is limited only to cases when a color original image is a positive image.
In addition, various methods in which the exposure amount for printing an original image recorded on a color negative film onto a light-sensitive material for printing is determined from a digital image obtained through image-reading a color original image with a scanner have been known. For example, Japanese Patent Publication Open to Public Inspection (hereinafter, referred to as Japanese Patent O.P.I. Publication) No. 46741/1980 discloses a method of correcting color from color original image based on a multiple regression equation calculated from each statistic of the density value difference between the B-G primary colors and the density value difference between the R-G primary colors each of which is statistic of neutral color density compared to the density value of each primary color of the original pixels.
However, the invention described in Japanese Patent O.P.I. Publication No. 46741/1980 calculates statistics of density value difference between B-G primary colors and R-G primary colors on neutral density value from the density value of each primary color of the original pixels initially. Therefore, the statistics are influenced by high saturation pixels. Even if the resulting multiple regression equation is used, there is a problem that favorable color balance correction can not be conducted. In addition, even though the high saturation pixels are attempted to be removed using aforesaid multiple regression equation, if the ratio of the high saturation pixels in the image on a frame of a color negative film is great, aforesaid high saturation pixels and the influence thereof cannot be removed.
In addition, Japanese Patent O.P.I. Publication No. 220760/1984 compares the saturation based on the difference of density value with the density value of a portion where the density value is the lowest in a film strip in pixels measured with a value set in advance. Aforesaid invention describes that, if the degree of saturation is less than a value set in advance, it is extracted as a neutral pixel, and that neutral pixels are extracted based on a value set in advance.
However, the invention described in Japanese Patent O.P.I. Publication No. 220760/1984 is based on the premise that the lowest-density value portion in the film strip is neutral. Therefore, if an object image portion having high saturation is the lowest-density value portion, this is selected so that an object image portion having high saturation becomes the standard of neutral. For example, in the case of a color original image photographed in a forest, the lowest density value portion is influenced by the dense green leaf image portion. Accordingly, an object image portion having high saturation is influenced so as to be a neutral standard and favorable color correction is interfered with.
In order to prevent the above-mentioned problems, it is possible to sense whether the lowest-density value portion in the film strip is located in a range set in advance, and only when it is within the range set in advance, aforesaid film can be used. However, in such cases, a low density value and high saturation pixels may be overly included. As a result, favorable color correction may be interfered with.
Incidentally, the inventions disclosed in Japanese Patent O.P.I. Publication Nos. 220780/1984 and 46741/1980 relate to photographic printing. These publications, however, do not disclose methods to correct the color from color original digital images.